The Latin title Rex Catholicissimus, rendered as Most Catholic King and Most Catholic Majesty was awarded by the Pope to the sovereigns of Spain. It was first used by Pope Alexander VI in the papal bullInter caetera in 1493.

One of the rights of a "Most Catholic" queen – either regnant or consort – is the privilège du blanc, meaning that she may wear white when meeting the Pope rather than the normal black used by other consorts and heads of state.